Catalytic comparison of various polar Zn(II)-Schiff base complexes and VO(II)-Schiff base complexes in (ep)oxidation processes of 1,2-cyclohexene and cyclohexane

The reactivity of Zn2+ and VO2+ ions towards pyridinyl Schiff bases, in the absence or presence of a p-sodium sulfonate group (HPSNa and HPS, respectively), provided polar and less polar catalyst complexes, which were characterized by various measurements. Stability constants of Zn-complexes [Zn(PS)2 and Zn(PSNa)2], and VO-complexes [VO(PS)2 and VO(PSNa)2] were measured spectrophotometrically. A typical (ep)oxidation processes of 1,2-cyclohexene or cyclohexane using 30% aqueous H2O2 catalyzed by our synthesized catalyst system, were tested. VO-complexes were found to be more effective catalysts than Zn-chelates. The polarity of Zn- and VO-complexes, i.e. the presence of p-SO3Na, displayed an observable influence on their catalytic performance chemoselectively. The polar catalyst system, Zn(PSNa)2 and VO(PSNa)2, in polar solvents such as H2O, MeOH, acetonitrile and acetone exhibited higher catalytic activity towards the (ep)oxidation processes than the less polar catalysts, Zn(PS)2 and VO(PS)2. In a less polar solvent such as CHCl3, polar catalysts showed higher conversion, but low chemoselectively, whereas the less polar catalysts showed relatively higher conversion and chemoselectivity. Under solvent-free conditions, less polar complexes were found to be more efficient catalysts than the polar chelates.